How does a CV carb work?
Posted: Fri May 05, 2006 11:53 am
I've got heavy experience tuning Holley carbs for performance cars. But I don't know how a Constant Velocity (CV, motorcycle) carb works. Specifically the vacuum chamber/piston/needle assembly.
Just looking at an exploded diagram, I *suspect* that the spring-loaded piston partially blocks the air path thru the carb, and as the throttle is opened and flow increases, the pressure above the diaphragm drops (due to the hole in the piston) and pulls the piston/needle up, reducing the velocity of the flow (hence CV), and simultaneously richening the mixture to handle the higher power.
The reason I ask....on my 95 CB750, when I give it full throttle, then let off the throttle slightly, I can feel a slight power increase. To me, this means it's just a little too rich under full throttle (would be true if it was a Holley). There's no surging or backfiring, and it "feels" fine at full throttle, just like a slightly rich/safe mixture would. My bike is all stock.
If my understanding of the CV function is correct, I need a needle with several notches that can be lowered slightly to lean the high-power mixture.
I just don't want to be making a wrong assumption about the CV carb function. I probably won't even bother to fine-tune it, just want to understand what's happening. Thanks in advance.
Just looking at an exploded diagram, I *suspect* that the spring-loaded piston partially blocks the air path thru the carb, and as the throttle is opened and flow increases, the pressure above the diaphragm drops (due to the hole in the piston) and pulls the piston/needle up, reducing the velocity of the flow (hence CV), and simultaneously richening the mixture to handle the higher power.
The reason I ask....on my 95 CB750, when I give it full throttle, then let off the throttle slightly, I can feel a slight power increase. To me, this means it's just a little too rich under full throttle (would be true if it was a Holley). There's no surging or backfiring, and it "feels" fine at full throttle, just like a slightly rich/safe mixture would. My bike is all stock.
If my understanding of the CV function is correct, I need a needle with several notches that can be lowered slightly to lean the high-power mixture.
I just don't want to be making a wrong assumption about the CV carb function. I probably won't even bother to fine-tune it, just want to understand what's happening. Thanks in advance.